If the above is possible where would you expect to see MAC addresses in the network?
Before I can answer your question there are a few principles that you need to be aware of, which will help you answer your question.
MAC addresses are used within local networks in order to uniquely identify each host. No two MAC addresses can be the same within a LAN.
When one computer needs to communicate with another, it requires two important pieces of information:
- The receiving computer's MAC address
- The receiving computer's IP Address
With these it is able to construct a frame which is placed on the network and arrives to the receiving end.
Again, the above rule applies to Local Area Networks.
When we move off our networking to the Internet or different network, we are talking about crossing WAN's, that is, Wide Area Networks.
The rules for WAN's are different from those of LAN's, and this is because different protocols are used in each case.
Some examples of LAN protocols are: Ethernet, Token Ring, FDDI etc. Examples of WAN protocols are: ISDN, Frame Relay, ATM etc.
The MAC address is only used within LAN's, which means when the frame arrives to the router in order to be forwarded to the appropriate remote LAN, all Layer 2 information (meaning the datalink layer, which is where MAC Addresses exist) is stripped and encapsulated in a different frame that the WAN network uses.
Frame Relay creates virtual circuits between two points. These virtual circuits can be permanent (also known as PVC's – Permanent Virtual Circuits) or switched (also known as SVC – Switched Virtual Circuits).
When you connect to a Frame relay, you need to know the DLCI (DataLink Connection Identifier), which isn't exactly the equivalent to the MAC address we know. The DLCI number allows your DTE (Data Terminal Equipment - your router in this case) to identify which virtual circuit it needs to use in order to send a packet to a remote network.
For example, let's consider the following network diagram:
|------------------| Router------DLCI 100----| Frame Relay |--DLCI 110—Router(10.0.0.1) | | Switch | | | & Cloud | |---DLCI 200----|------------------|--DLCI 250—Router(220.127.116.11)In this setup, our router on the left hand side connects to two remote networks via frame relay. You can clearly see that each remote network (10.0.0.0 & 18.104.22.168) has a virtual connection to the frame relay cloud, and our router (on the left) has two connections, one for each remote network.
In order for our router to distinguish between the two remote networks, it creates a mapping, that is, a mapping of the local DLCI and the remote network IP.
For example, if we assumed that the DLCI 100 connects to the 10.0.0.0 network and DLCI 200 connects to the 22.214.171.124 network, this would be the mapping our router would create:
DLCI 100 ---> to 10.0.0.0 network DLCI 200 ---> to 126.96.36.199 network
The above illustration is very rough, but contains enough details to help you understand how the DLCI is used.
I hope I've helped clear all possible questions!
This was first published in August 2003